1. Field of the Invention
The present invention relates to a non-contact IC card, and more particularly to the construction of a non-contact IC card which is capable of controlling power consumption and a method of using the same.
2. Description of the Related Art
In recent years, among the various IC cards, attention has focused on non-contact IC cards which do not have exterior electrodes on the surfaces thereof. Non-contact IC cards send and receive signals among external devices in the same way as ordinary IC cards having exterior electrodes. These signals are sent and received using space transmission media such as electromagnetic waves, light, or magnetism.
The construction of a conventional non-contact IC card is shown in FIG. 7. ROM 2 and RAM 3 are connected via a bus 8 to a CPU 1 that controls the operation of the IC card. An input/output control circuit 4 that controls the inputting and outputting of data with external devices is connected to the bus 8. An antenna 6 is connected to the input/output control circuit 4 via a modulation/demodulation circuit 5. In addition, a battery 7 for supplying power to the respective electrical circuits is built into the IC card.
In such an IC card, when a request signal from an external device is received by the antenna 6, this request signal is demodulated by the modulation/demodulation circuit 5, and then is input to the CPU 1 via the input/output control circuit 4. The CPU 1 deciphers the request signal and generates a predetermined response signal. This response signal is input to the modulation/demodulation circuit 5 via the input/output control circuit 4. After the signal is modulated by the modulation/demodulation circuit 5, it is transmitted to an external device from the antenna 6.
FIG. 8 shows the construction of a demodulation circuit section of the modulation/demodulation circuit 5. The antenna 6 is connected to the positive input terminal of a comparator 51; A reference voltage generation circuit 52 is connected to the negative input terminal. A waveform shaping circuit 53 is connected to the output terminal of this comparator 51. A signal received by the antenna 6 is compared with a reference voltage generated by the reference voltage generation circuit 52. When the signal received has a voltage value higher than the reference voltage, a signal corresponding to the signal received is output from the comparator 51. After this output signal is shaped to a predetermined waveform by the waveform shaping circuit 53, it is transmitted to the input/output control circuit 4 as received data.
As shown in FIG. 8, however, since the reference voltage genration circuit 52 within the modulation/demodulation circuit 5 generates a reference voltage by dividing the power-supply voltage with resistors 54 and 55, current flows through these resistors 54 and 55 at all times. Therefore, a problem arises in that even when the card is not used, such as during storage, the battery 7 is consumed, and the actual service life of an IC card is shortened depending upon the length of the storage period before the card is issued.